Method and apparatus for non-invasive, cuffless, continuous blood pressure determination

Abstract

Arterial blood pressure of a subject is determined by detecting the EKG for the subject and selecting a fiducial point on the EKG during a pulse. Apparatus is provided for monitoring blood volume versus time at a selected location on the subject's body such as a fingertip. A time difference between the occurrence of the selected fiducial point on the EKG and a selected change in blood in volume at the selected body location is determined. This time difference depends on the arrival time of the pulse at the distal location in addition to the shape of the blood volume versus time curve. Heart rate is determined from the EKG. The arterial pressure is computed from pulse arrival time, volumetric wave shape and instantaneous heart rate for each pulse. It is preferred that the fiducial point on the EKG be an R-wave. A suitable method for determining change in blood volume utilizes photoplethysmography. Methods are disclosed for determining diastolic pressure, systolic and mean arterial pressure. In another aspect, artifact detection and rejection enabled. The invention provides for a continuous measure of blood pressure in a non-invasive, cuffless manner.

What is claimed is: 
     
       1. Method for determining arterial blood pressure in a subject comprising: detecting an EKG signal for the subject for a series of pulses in a time window;   selecting a fiducial point for each pulse on the EKG signal;   monitoring blood volume versus time waveshape at a selected location on the subject's body for the series of pulses;   determining instantaneous heart rate for each pulse from the EKG signal;   calculating arterial pressure from the instantaneous heart rate and the blood volume versus time waveshape for each pulse;   sorting by value a function of at least one of the EKG signal and the blood volume versus time waveshape for each pulse over the series of pulses   calculating a parameter based on the sorted values; and   detecting artifacts from the calculated parameter.   
     
     
       2. The method of claim 1 wherein the fiducial point is a point on an R-wave. 
     
     
       3. The method of claim 2 wherein the fiducial point is the peak of the R-wave. 
     
     
       4. The method of claim 1 wherein the calculating step includes utilizing a selected change in blood volume on the blood volume versus time waveshape. 
     
     
       5. The method of claim 4 wherein the selected change in blood volume is in the range of 20% to 80% on an upslope on the wave shape. 
     
     
       6. The method of claim 4 wherein the selected change in blood volume is in the range of 40% to 60% on an upslope on the wave shape. 
     
     
       7. The method of claim 4 wherein the selected change in blood volume is approximately 50% on an upslope on the wave shape. 
     
     
       8. The method of claim 1 wherein the selected body portion is a distal location. 
     
     
       9. The method of claim 8 wherein the distal location is a fingertip. 
     
     
       10. The method of claim 1 wherein monitoring blood volume versus time comprises utilizing photoplethysmography. 
     
     
       11. Method for determining arterial blood pressure in a subject comprising: detecting an EKG for the subject for a series of pulses in a time window,   selecting a fiducial point for each pulse on the EKG signal;   monitoring blood volume versus time at a selected location on the subject's body for the series of pulses;   determining time difference between occurrence of the selected fiducial point and occurrence of a selected change in blood volume at the selected body location for each pulse;   determining heart rate for each pulse from the EKG; and   computing arterial pressure based on the time difference and heart rate for each pulse;   sorting by value a function of at least one of the EKG signal and the blood volume versus time waveshape for each pulse over the series of pulses   calculating a parameter based on the sorted values; and   detecting artifacts from the calculated parameter.   
     
     
       12. The method of claim 11 wherein the fiducial point is a point on an R-wave. 
     
     
       13. The method of claim 12 wherein the fiducial point is the peak of the R-wave. 
     
     
       14. The method of claim 11 wherein selected change in blood volume is in the range of 20% to 80%. 
     
     
       15. The method of claim 11 wherein selected change in blood volume is in the range of 40% to 60%. 
     
     
       16. The method of claim 11 wherein selected change in blood volume is approximately 50%. 
     
     
       17. The method of claim 11 wherein the selected body portion is a distal location. 
     
     
       18. The method of claim 17 wherein the distal location is a fingertip. 
     
     
       19. The method of claim 11 wherein monitoring blood volume versus time comprises utilizing photoplethysmography. 
     
     
       20. The method of claim 11 wherein arterial pressure is diastolic pressure. 
     
     
       21. The method of claim 11 wherein arterial pressure is systolic pressure. 
     
     
       22. The method of claim 11 wherein arterial pressure is mean arterial pressure. 
     
     
       23. The method of claim 22 wherein mean arterial pressure, P M (i), is determined by computing   P.sub.M(i) =(P.sub.S(i) -P.sub.D(i))*1/3+P.sub.D(i)     where P S (i) is systolic pressure, P D (i) is diastolic pressure, and i refers to the i th  pulse.   
     
     
       24. Method for determining diastolic blood pressure in a subject comprising: detecting an EKG for the subject:   selecting a fiducial point on the EKG during a pulse;   monitoring blood volume versus time at a selected location on the subject's body.   determining time difference between occurrence of the selected fiducial point and occurrence of a selected change in blood volume at the selected body location;   determining hear rate from the EKG; and   computing diastolic pressure based on the time difference and heart rate, wherein diastolic pressure, P D (i), is determined by computing   P.sub.D(i) =(K.sub.Dv *v.sub.p(i).sup.2)+(K.sub.Dihr *IHR.sub.(i))+K.sub.Dcal       v.sub.p(i).sup.2 =(1/T.sub.R--50(i))*(1/T.sub.R--50(i)     where T R--50 (i) is the time difference between occurrence of the selected fiducial point and a change in blood volume at the selected body location; K Dv  and K Dihr  are constants and K Dcal  is a calibration constant; IHR.sub.(i) is the instantaneous heart rate; v p (i) is the pulse velocity; and i refers to the i th  pulse.     
     
     
       25. The method of claim 24 wherein K Dv  is approximately 2.5 and K Dihr  is approximately 0.5. 
     
     
       26. Method for determining systolic blood pressure in a subject comprising: detecting an EKG for the subject;   selecting a fiducial point on the EKG during a pulse;   monitoring blood volume versus time at a selected location on the subject's body;   determining time difference between occurrence of the selected fiducial point and occurrence of a selected change in blood volume at the selected body location;   determining heart rate from the EKG; and   computing systolic pressure based on the time difference and heart rate, wherein systolic pressure, P S (i), is determined by computing   P.sub.S(i) =(K.sub.Scal *v.sub.p(i).sup.2)+K.sub.Sconst       wherein K Sconst  is a constant; K Scal  is a calibration constant; v p (i) is the pulse velocity; and i refers to the i th  pulse.   
     
     
       27. The method of claim 26 wherein K Sconst  is approximately 35. 
     
     
       28. Method for determining mean arterial blood pressure in a subject, comprising: detecting EKG for the subject;   selecting a fiducial point on the EKG during a pulse;   monitoring blood volume versus time at a selected location on the subject's body;   determining time difference between occurrence of the selected fiducial point and occurrence of a selected change in blood volume at the selected body location;   determining heart rate form the EKG; and   computing mean arterial pressure based on the time difference and heart rate, wherein mean arterial pressure, P M (i), is determined by computing   P.sub.M(i) =((K.sub.Scal *v.sub.p(i).sup.2)+K.sub.Sconst -(K.sub.Dv *v.sub.p(i).sup.2)+(K.sub.Dihr *IHR.sub.(i))+K.sub.Dcal)*1/3+(K.sub.Dv *v.sub.p(i).sup.2)+(K.sub.Dihr *IHR.sub.(i))+K.sub.Dcal       wherein T R--50 (i) is the time difference between occurrence of the selected fiducial point and a change in blood volume at the selected body location; K Dv , K Dihr  and K Sconst  are constants and K Dcal  and K Scal  are calibration constants: IHR.sub.(i) is the instantaneous heart rate; v p (i) is the pulse velocity and i refers to the i th  pulse.   
     
     
       29. The method of claim 28 wherein K Dv , is approximately 2.5, K Dihr  is approximately 0.5, and K Sconst  is approximately 35.